/Engine/Plugins/Experimental/Mutable/Source/MutableRuntime/Public/MuR/Instance.h
Get an optional, opaque application-defined identifier for this surface. The meaning of this ID depends on each application, and it is specified when creating the source data that generates this surface. SeeNodeSurfaceNew::SetCustomID.
uint32 GetSurfaceCustomId ( int32 lod, int32 comp, int32 surf ) const